text:
Does the cannon of the A-10 deliver more thrust than the engines do?
The A-10 Warthog is built around a 30 mm cannon that fires depleted uranium rounds, 65 of them per second.

I heard from someone who worked on the flight controls a long time ago, that when the cannon is fired it provides more thrust than the engines. Is this true? What are the effects of this?
Update
The original question was answered by @Gypaets: thrust of the cannon is comparable to thrust of one engine. The wikipedia article on the GAU-8 cannon mentions that effects of thrust of the cannon are well accounted for by placing the firing barrel at the centre line of the air frame. Ron Gordon's answer reveals some interesting side effects from spooling the cannon up and down when operating - the aircraft bucks. It's a big cannon.

Image source

I've heard it produces more thrust than a single engine, not both.

mandatory link: https://what-if.xkcd.com/21/ (scroll towards the middle for the section on the GAU-8/A)

It's possible the thing you're forgetting is the cannons only produce force momentarily, the engines produce force continuously.

If the canon produces thrust equivalent to one engine, for 200 ms, then this is equivalent to power down an engine during 200 ms. No big deal I suppose, the vertical stabilizers should be able to cop with that, specially if the canon is not remote for the longitudinal axis.

@mins They fire for longer than 200 ms.

@Fattie Where did you get the impression I'm forgetting that?

@Koyovis true. But if you can't get it done in 200ms, then you're into spray-and-pray, in which case, I wouldn't count on another 600ms closing the deal.

I would think the real problem is not engine power, it is stall speed. The way I see it as a non military bystander is that the A10 is supposed to optimize attack time - so on a run it will come in low and SLOW. Slow means there is not a lot of reserve before hitting stall speed AND it means engines likely are not running full power. This makes it a balance act of keeping speed low but above stall speed, and balancing fire and engine output to just dance in the optimal speed range.

@jjack There is nothing dragging. It is thrust, pointed backwards.

@jjack It's called a thrust reverser because it reverses the direction of the thrust. Thrust is not bound to a direction of motion. A helicopter rotor produces thrust directed upwards, while it flies forwards. An aerobatic aircraft sliding backwards has still engines producing thrust, but now in the opposite direction of motion. Let me reverse the question: why is it called a thrust reverser and not a dragger?

Not only does it affect air speed, but also angle of attack (AoA) and yaw.  The gun is well forward of the center of gravity (CG); so add to the recoil vectoring a build up gyroscopic torque as the gun spins up and down.  As air speed decreases, maneuvering approaches instability, thus placing a limit on the length of a safe burst determined by available energy (altitude, AoA and air speed.)  However, grouping accuracy and ammo conservation usually are more limiting than the effect of gun "drag".  Each A10's gun performance is distinctive.   Once a burst begins, the aircraft will pitch and yaw in its peculiar manner.  On the HUD you might find a wad of gum placed there by the pilot after his first aiming burst to show the direction of push.
The A10 employment scenario has always included battle field loiter to provide air cover, ensure extended engagement coordination, and provide tactical intel directly to the ground commander! Even low and slow, its hard to hit. Most infantry borne G-to-A missiles are IR guided. The A10's exhaust is hidden from below by the horizontal stabilizer; and the RAW/IRW system can automatically launch "angel flares" when the launch is detected, thereby negating any IR guidance system. Meanwhile you have just poked the bear and left your location for immediate response with an array of launch-and-leave air-to-ground guided weapons.

Are pilots matched with airframes, so that they gain experience with one particular set of peculiarities? Can the A-10 continue to fly the mission on 1 engine?

Yes. Absolutely.  The pilot is a member of the assigned combat team for that aircraft, training, exercising, and deploying together.

@Koyovis: "Can the A-10 continue to fly the mission on 1 engine?" – The A-10's purpose is to fly low and slow over an active battlefield, it is designed for extreme survivability: "The aircraft is designed to fly with one engine, one half of tail, one elevator, and half of a wing missing."

Indeed, and there are quite some photo's of battle damaged aircraft that made it home ("You should have seen the other guy!"). But i'm wondering if the A-10 can continue the mission on one engine, or if it is a get home fast scenario.

Comments right on.  Your picture shows how the engines are protected from ground fire by control surfaces that will have to be shot away first. From the technical descriptions of thrust limitations you have elsewhere, an engine failure is usually a good reason for abort.  But the exigencies of war dictate risk taking proportional to the objective.  The A10 can loiter on one engine, providing surveillance and coordination until help comes.   It also carries an array of guided weapons that can be used, if the bomb/nav release systems are still functioning.

@Koyovis - I would assume definitely RTB. The A-10 isn't exactly sporty to begin with. Without a second engine, you lose altitude or jettison stores. AFAIK, that "one engine... half of a wing missing" etc is clean w/o stores or other damage. It's physically possible to do some missions one one engine, but I doubt it's advisable. That's my impression from pilot comments. The survivability features are designed to keep the pilot alive and bring the plane home, not let it keep fighting while shot up. Overall, it's the least survivable USAF fighter, not the most.

@HephaestusAetnaean The survivability features are designed to keep the pilot alive and bring the plane home, - sounds like some pretty sensible survivabilitynfeatures to me.

@Koyovis - Exactly! Sensible indeed. And it's the same mentality with lots of planes, not just the A-10. They're not expected to sponge bullets every mission. People get the idea the A-10 is an impervious flying tank, soaking up 30mm and bouncing bullets. That's not sensible. Everyone remembers this photo (https://en.wikipedia.org/wiki/File:Kim_campbell_damage_a10.jpg), but everyone forgets that that plane sat on the ramp for weeks waiting on repairs. They're aircraft, not tanks. Low n' slow and BRRRRT are romantic, but not very survivable in a high-threat environment.

@Federico The last bit added has no direct relevance to the question. It is interesting info though, and should belong in a comment.

"and the RAW/IRW system can automatically launch "angel flares" when the launch is detected, thereby negating any IR guidance system."  Modern IR-guided weapons are designed to ignore flares.

The A-10 uses a GAU-8/A. According to Wikipedia:

The average recoil force of the GAU-8/A is 10,000 pounds-force (45
  kN), which is slightly more than the output of one of the A-10's two
  TF34 engines (9,065 lbf / 40.3 kN each). While this recoil force is
  significant, in practice a cannon fire burst only slows the aircraft a
  few miles per hour in level flight

Your statement is only partially true: two engines have more, but one engine has about 10% less thrust than the recoil force of the cannon.
The recoil force of MiG-27's GSh-6-30 was even larger: 60 kN.
As pointed out in a comment by @tj1000 the recoil of the GAU-8/A wasn't such a problem as the smoke from its shell propellant. If you're looking for the video mentioned in the comment (test pilot ejection due to engines out after firing ammunition with a new flash suppressor) this is the link (12:08-13:59).

Can it still fire the gun when flying on one engine?

@Koyovis I suppose it is more or less like airebrakes: it reduces the total energy, so yes you can still fly but not maintain a leveled flight at constant speed.

And perhaps struggle with the yawing moment.

The big problem with the GAU-8 and the A10 during development wasn't recoil, it was the smoke from the cannon shell propellant blowing the engines out. If you've ever seen the oft repeated video of a pilot punching out of an A10, and the aircraft then crashing into a desert, that's what happened: during a test, the cannon smoke blew out both engines, and the pilot couldn't get them restarted.

@tj1000 Thanks for the comment! I found the video and added a link to it.

I first saw that film around 1980, in Air Force ROTC training. This was during extended testing of the A10. When the pilot got below 2000 feet and the engines still hadn't restarted, they told him to get out. They showed that film as it was shot on a high speed camera (to record the cannon firing), and it was a good example of an ejection. They pointed out that as the pilot didn't have his head completely vertical, he did get a cracked vertebrae from the ejection, but no permanent damage.

That's a great video.

@tj1000: I'm surprised that would be enough to flame out the engines, given that, even if they are sucking in smoke, the vast majority of what they're ingesting is still air.

@Sean That's an interesting point. Would you like to ask a separate question about what needs to happen to flame out such a jet engine?

The GAU-8 is in line with the engines, insofar as airflow goes, so any gas from the cannon goes straight back to the engines. Given the rate that the GAU-8 fires, and the size of its shells, it appears that it puts out a LOT of smoke. There was a similar problem with the first rotary cannon, a 20mm Vulcan fitted to the F104. The blast damaged the area around the cannon, and seriously reduced engine power. Eventually, they fixed that problem and the Vulcan became standard on a lot of warplanes.

As Gyrates points out, no. The recoil from the GAU-8 doesn't completely counteract the thrust of both of the A10's TF-34 engines, but it will definitely slow the plane down. That is a factor before firing the cannon - is your airspeed sufficient to withstand a slowdown?
The same holds true of WW2 fighter aircraft with multiple heavy weapons - firing at slower speeds could stall the plane.
One airplane that did battle serious recoil issues was the WW2 B25 variant that mounted a 75mm cannon in the nose. Apparently, the recoil from the cannon was quite severe, and definitely impacted airspeed.
AC130 crew report that firing the side mounted 105mm howitzer will boot the tail of the plane around quite a bit.

Well said. I would think - as a non pilot, jsut from some logic - that part of the problem is that on an attack run the A10 would come in low and SLOW - to give more time for the run. Which means there is not a lot of reserve before hitting stall speed. And the engines possibly are not working at full power either. Which means the question is really "does the gun slow down the plane significantly". Engine power does not really matter in a scenario I describe.

Fortunately, most A10 attacks with the gun are made with the plane in a slight dive - partially to keep the target centered, and partially to keep a reserve of speed - acceleration is quicker when a plane is already diving. I don't think the GAU8 has to stay on target for very long to destroy whatever it is aimed at.

Yeah, but I think someone envisioned a battlefield with a tank battle going on and an A10 slowly going over the battlefield obliterating a dozen or so tanks. Not that this is the case these days ;)

Oh, yes! two A10s, 23 tank kills, one day. .https://theaviationist.com/2012/12/20/warthog-in-action/  And its not just the plane!!  These guys used a dirt strip to refuel and reload.  We practiced ground turn around on the A10 bring it down to less than 15 minutes without peacetime safety restricts prohibiting loading bombs and fuel simultaneously.  In this scenario, the pilot is the guy pump his on JP fuel.

A figure I saw was that the 8 M2's on a P47 would put the equivalent of around 400hp in reverse. Not enough to stop the plane, but definitely a factor. If the aircraft was close to stalling in a drastic maneuver, firing the guns could push it into a stall.


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Can I fly from the UK to Spain on an NPPL(A)?
I have a UK-issued NPPL(A) with microlight rating, can I fly my own aircraft to Spain on this?

The NPPL is a UK only license and is not recognized by other countries, however there is a letter of agreement between the UK and France that you can fly there provided you have a medical. There is no such agreement with Spain, however you can contact their version of the CAA and maybe get permission, I'm not sure what hoops you would need to jump through on that.

A chap on the Microlight Forum helpfully phoned AESA to ask:

The Spanish Agency (AESA) [...] confirm that to fly a microlight in
  Spain you need a Class 2 or a LAPL medical certificate. The LAPL
  certificate is the least they accept, and must be issued by an
  aeromedical examiner.

Source: Microlight Forum: Taking permit aircraft to Spain on an NPPL
The  CAA document Which type of Medical Certificate or Declaration can I use for my licence? is also kind of useful in working out what the options are, if not actually giving the answer for individual countries.

source: Stack Exchange

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sort: votes


text:
What's Concorde's engine fan blade tip speed at full thrust?
I can't find anything on the web about the Concorde's Snecma engine fan blade tip speed at full thrust. Any ideas?

Maybe because there is no fan as in a turbofan.

Apart from the fact that the Rolls-Royce/Snecma Olympus 593 engines were pure turbojets so they did not have any fan, it's pretty easy to calculate (better approximate) the tip speed of the first Low-Pressure Compressor's wheel.

The Olympus 593 had a diameter of about 1.2 meters [Wikipedia]. Let's say the first compressor wheel had a diameter of about 1 meter.
As we're talking about the LP Compressor, we're interested in N1 speed. At 100% N1 speed, we are at 6500 rpms [FAA Type Certificate No. E20EU] (for N2, at 100%, 8530 rpms).
Now it's just math to calculate, or, just use an online converter.

The result, as DoRoCro already wrote in his answer, is about 340 m/s.

As a first approximation, about Mach 1 (for the incoming air at the fan blade row, depending on temperature in turn depending on altitude) - any higher and you generate shock waves off the blade which inhibits the flow.  The blade tip might be slightly above this so that some of the outer part of the blade is supersonic, but outside my knowledge when it comes to design of transonic compressors, but my guess is given the age that the design was subsonic.
(Remember that the job of the intake is to slow the incoming supersonic airflow down to speeds that the compressor can handle, at the same time boosting the pressure of the air ahead of the first fan stage)
http://www.saveconcordegroup.co.uk/engines/ suggests axial flow at the engine face is about 0.49 Mach

I believe the OP is not looking for the axial velocity but the velocity at the leading edge of an hypothetic fan blade tip (sum of the velocities W1).

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Why are the aerodynamic characteristics of a finite wing different from the properties of its airfoil section?
Why are the aerodynamic characteristics of a finite wing different from the properties of its airfoil section?

Can you clarify what you have in mind by giving an example of a characteristic different between wing and airfoil?

To be honest I dont really know...this was a question from my previous exam but I suppose it's related with lift and moment coefficient, for example

The airfoil is a 2D representation of a wing slice, but the wing is a 3D object, non rectangular, usually swept and twisted, with different airfoils from root to tip, with non constant angle of attack. Wing characteristics are "composite".

Because the wing is finite, while the airfoil is assumed to stretch out sideways indefinitely. The consequence is that on a wing you get spanwise flow outside of the plane of symmetry, while on the airfoil all flow speeds are in the airfoil's plane only.
Spanwise flow is caused by the pressure difference between lower and upper side which can equalise around the wingtips. This lets the pressure difference diminish towards the tips and especially in the rear part of the wingtips. The consequences are:

Lift is smaller on a "real" wing relative to a wing of the same airfoil with no such sideways flow.
The center of pressure of a "real" wing is ahead of the one of the airfoil.
On a swept wing the air transverses the surface differently than it transverses the airfoil. Basically, a sideways speed component is added to the speed around the airfoil.

Thank you so much for your help Peter!!!
You made my day!


text:
What is the standard protocol for landing on the ocean?
If a plane had problems whilst flying and had to land in the ocean, what is standard protocol for pilots and cabin crew to follow in order to try and keep everyone safe?

For pilots: Are airliners' pilots trained to safely ditch an aircraft?

You might want to check the ditching questions we already have, especially this one and this one.

The first step to safe flying is don't ditch.

This appears to be a dupe of those two other questions.


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Are there any precision approaches that use only GNSS/LAAS?
Is there any airport which has a precision instrument approach using only GNSS (augmented with LAAS) for guidance?

Are RNP approaches precision?

The FAA has an FAQ on GBAS (ground-based augmentation of GPS/GNSS). It says they no longer use the term LAAS, instead they now use GBAS to be consistent with ICAO terminology. A GBAS approach is called GLS on the approach charts.
The FAQ has information about where GBAS is used:

Where are operational GBAS systems located?
Currently, two U.S. locations have obtained operational approval for
  public GBAS use and support revenue airline traffic. These stations
  are located at Newark Liberty International Airport (EWR) and Houston
  George Bush Intercontinental Airport (IAH). Internationally, several
  airports have GBAS and support revenue operations (Bremen and
  Frankfurt, Germany; Sydney, Australia; Malaga, Spain; Zurich,
  Switzerland; and 15 Russian locations).

Newark (KEWR) has 5 GLS approaches, Houston (KIAH) has 6. I looked at a couple of the approach plates and they didn't require any additional equipment apart from the GBAS equipment itself. Note that GBAS is not the same as the 'typical' GPS/WAAS combination used for RNAV GPS approaches, and it requires different equipment:

Can the same avionics be used for GBAS and WAAS?
The GBAS and WAAS standards are different and initial avionics and
  integrations only supported one of the two capabilities. Emerging
  aircraft integrations can support both services (e.g. A350).

you wrote "Note that GBAS is not the same as the typical GPS/WAAS combination used for RNAV GPS approaches " ; I am a bit confuse, GPS/WAAS cannot be used for precision approaches right? RNAV approaches using GPS MUST be GBAS to be considered precision approaches, am I right?

@Ghilardi It depends what you mean by "precision". See this answer but it looks like ICAO has a somewhat confusing definition that doesn't include LPV. However, at least in my GA experience, most pilots use "precision" to mean simply "with vertical guidance", which does include LPV.

yes, I have read it and unfortunately still confuse anyway. I will dig deeper in that. Thank's @Pondlife

According to the current FAA/ICAO definitions,  GLS (i.e. with GBAS) precision approaches are completely separate from RNAV (with or without SBAS) non-precision approaches. In practice, ILS Cat II/III is a completely different beast from ILS Cat I, and the split between GLS and RNAV looks like it'll be similar if not worse.

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text:
How important are my EASA exam scores for getting a future airline job?
I'm doing my ATPL and I hear a lot of stories about what scores you should get in your exams to get a job as soon you get the licence. Does anyone have any information about how important the exam scores really are for a future job?

I'm not sure if employers have access to the scores, but like any other "just out of diapers" jobs, if the only thing an employer has to choose between you and Joe Perfect is your test score, how do you think the employer will make that decision? May not keep you from getting a job, but it may mean the difference between working for Emirates or some run down regional...

There is no real fixed answer to this as each airline has its own requirements, but generally speaking the better your results the better your chances. As one example, Air New Zealand list their policy on their website, which focuses on the amount of times you have to resist the exam: https://careers.airnz.co.nz/explore-our-careers/career-areas/more-about-becoming-a-pilot .


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What are the consequences of fouled spark plugs in flight, and how can I clean them?
I've experienced spark plug fouling before but it was during the pre-flight checks and my instructor fixed it for me (by using max RPM and leaning the mixture a couple of times). But now that I have to fly all by myself, I'd rather know what to do and what not to do, especially during flight. What happens if the plugs become fouled in the air, and how do I clean them in flight?

When I was first starting to fly, I often had fouled plugs. After a few expensive plug cleanings, I learned about aggressive leaning. When on the ground lean very aggressively—so much that if you give the plane full power that it will stall. I also do my mag check leaned out. When I am ready to make my call to the tower (or CTAF) I put my hand on the mixture and don’t move it until I get cleared for takeoff.

The only time I have had fouled plugs in the 15 years since then is one time when I had a slow 2 hour flight in a Cherokee and did not lean enough. I did not notice any problems in the air, but when I did my mag check before shutting down, I noticed that the plugs were fouled.

You should read up here but generally speaking plugs are less likely to foul in flight (and is why its not talked about much). Plugs often foul due to the mix of low engine temps and high led content during warm up at full rich settings. This is why a leaned out warm up is advised for some planes, you should talk with your instructor about the plane you are flying and what the proper warm up procedures are.
Please consult your POH for any airframe specific emergency checklists that may apply to your aircraft. Generally speaking if you depart with a fouled plug and it is causing a rough running issue you should cycle the magnetos untill you are running on the good set of plugs and get the plane down safely. This is most likely the result of a poor or non-existant run up check. Unless you are sitting in line for a long time after your runup, you should generally be doing runup close to departure time.

The best answer here is don't use an excessively rich mixture.  That will avoid a lot of fouling problems on the plugs.  Leaning during taxiing also helps.  During flight betweeen SL and approx 4000 FT ASL, at full throttle lean the engine until the fuel flow indicator is at the top of the green arc.  Above that level, do not operate the engine richer than 70 deg Rich Of Peak.

This isn't good advice unless you have a fuel flow meter and EGT gauge. Leaning without them can be problematic so a lot of schools don't teach it, but it's an important skill to learn.

You can actually lean an engine pretty well by sound and Bu feel.  In addition most modern light aircraft are equipped with an EGT gauge.

@CarloFelicione is right. In fact, some Cessna manuals for Lycoming O-320s call for leaning the engine till it gets rough and then enriching the mixture till the roughness goes away. Fancy instrumentation is not necessarily required to run an engine well.

I agree that leaning is a good idea, and that it can be done by listening for an rpm drop. This is how I do it because I rarely ever see an airplane with a working EGT.


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What is a flat rated engine?
I've heard that a "Flat Rated Engine" is an engine where the power is reduced to keep the internal temperature within certain limits when the ambient temperature is above a certain point. Is that correct? How does it work?

well.. I didn't get clearly what is a flat rated engine and how does it work.

The engine doesn't measure OAT (at least not in general, some engines with FADEC can). Lower density air has less air running over the cooling part, which in aircraft are almost all air-cooled. The less air running over the parts affects cooling, so you have to reduce power to help keep the engine cool. A "flat rated" engine is rated to provide maximum take-off power through the entire climb. My 177-B is "flat rated" but I reduce power at 500' anyway to help cooling (and so I can close my cowl flaps).

Yes, a flat rated engine (my familiarity is with gas turbines, but the concept may apply to other engine types), is an engine for which the control system is designed to achieve the same level of thrust (hence the "flat" term), up to a certain OAT, then for higher ambient temperatures, the level of thrust decreases as a function of OAT, normally in a pretty linear fashion. This control system operation gives identical performance over the flat portion of the OAT range even while the engine degrades (until it degrades to the point it hits the EGT limit), making the plane performance predictable. But for higher OATs, it stops the EGT continuing to increase. The temperature at which the thrust starts to decrease is typically 25 or 30 deg C, and can be found in the EASA or FAA engine type certificates (e.g. see section 6 of this EASA one for the CFM56-5). See the graph from this CFM56 document below. So, your understanding was correct.

Flat rating is a concept by which an engine, as stated above is restricted, in terms of power or thrust output by the fuel control unit to produce a uniform power or thrust output as the aircraft climbs or descends until the ambient air conditions will not allow the engine to operate at or beyond the maximum thermodynamic output of the engine.  Like any other heat engine, the power or thrust produced by an aviation gas turbine (the gas core) depends on the available mass flow rate of working fluid (air in this case) as well as the operating temperature limits of the engine's hot section.  The maximum available power or thrust a gas core can produce at sea level at STP is referred to as the core's thermodynamic rating.  As an example, the Pratt & Whitney Canada PT6-66D turboshaft engine has a thermodynamic rating of 1,850 horsepower, but the FCU flat rates this to a constant maximum power output from the driveshaft to 850 ESHP, which remains available all the way up to approx FL260, thereafter decreasing with an increase in altitude.

This may be a good working definition

The Flat Rating Concept
The rating of a jet engine is the thrust performance that is
  guaranteed by the manufacturer for a new engine under specific
  operating conditions such as, take off, maximum continuous, climb,
  cruise.
Some non Prat and Whitney (P&W) engines are rated to a constant
  compressor speed (RPM). P&W rates there engines to a constant exhaust
  gas temperature (EGT). This is referred to as the flat rating concept.
  To get your head around this concept you must understand the following
  principle.
The temperature and density of the ambient air vary inversely.
Lower temperature = > Higher Density
Higher Temperature = >Lower Density
The amount of airflow (lbs/sec) through the engine is a function of
  compressor speed and air density. It is greater when the compressor
  speed and density are high.
The compressor speed is a function of the energy available to the
  compressors turbine. That energy comes from the combustion or air and
  fuel, so the turbine turns faster when the fuel flow rate is greater.
  The compressor speed is also a function of the airflow through the
  compressor. Higher rates of air flow reduce the speed of the
  compressor.
The compressors rotational speed and the amount of airflow through the
  compressor are independent, they affect each other though.
The turbine inlet temperature is proportional to the energy available
  to turn the turbine. The exhaust temperature is proportional to the
  turbine inlet temperature. So a higher EGT corresponds to a larger
  amount of energy to the turbine so it can turn the compressor faster.
When EGT is held constant, or lowered the result is a prolonged hot
  section life and at the same time provides the thrust to meet the
  certification requirements.

Or a bit more concise from wiki

When an engine is Flat rated it means that an engine of high
  Horsepower rating is constrained to a lower horsepower rating. The
  engine output in this case will always remain the same, but when
  atmospheric conditions such as high temperatures and high altitude
  (Hot and High) reduce the power output of the engine it has more
  headroom before it falls below the limited maximum output.

Just to check I understand, could it therefore be said that when atmospheric conditions are below ideal, the engine still performs to spec because it was previously limited by EGT and not atmospheric conditions?

@Notts90, almost. The engine always performs to spec, because spec just describes how the engine behaves. The point is that the performance is such that the engine produces the same thrust and power up to certain density altitude (above it the engine becomes RPM-limited and the power decreases).


text:
Does EASA require airliners to be fitted with autopilots?
For EASA OPS requirements, must an airline transport airplane be fitted with an autopilot system?

I'm an ATPL student in Eu and during my study I have read that for FWS (Flight warning System) It is a requirement for EASA OPS to have an ALTITUDE ALERTING SYSTEM for all turbjet aircraft and turbine powered ac weighing more than 5700kg or has more than 9 seats. But I have not found nothing about the requirements for A/P for aircraft.

Indeed EASA requires "autopilot" but they don't state it directly like "every airliner flying in the European airspace must be autopilot equipped". They require it indirectly.
When I first read your post I immediately thought of RVSM. I recall talking to an ATCO (retired now) about automation on board, and he told me that without autopilot you cannot fly in RVSM airspace.
So I had a look in skybrary concerning RVSM and they did the dirty job for us. They mention all 3 EASA regulations  IR-OPS SPA.RVSM.100 and SPA.RVSM.110, EU-OPS 1.241, EU-OPS 1.872 and they have quoted the interesting parts:

Regulatory Requirements[...]
An operator shall ensure that aeroplanes operated in RVSM airspace are equipped with:[...]
An automatic altitude control system

Emphasis is mine.
There might be other regulations requiring other automatic flight components but I am not aware of them although a quick look in the documents reveals references to auto-land systems as well so these documents cover more than RVSM.
The summary is that EASA does not simply specify "you need autopilot if you fly an airliner" but they go into detail, of what they require and for what reason.

Acronyms
RVSM Reduced Vertical Separation Minima
ATCO Air Traffic Control Officer

This answer could be improved by expanding RVSM at least once.

@ErinAnne hope it's better now :)

Absolutely!  Though you left out EASA from your acronyms (kidding, kidding)


text:
What is the flight time limitation for commercial air transport operation in Europe?
What is the maximum hours of flight time that a pilot can work legally in Europe? (daily, monthly and yearly)

For EASA, flight times and duty periods are limited (PDF p.25) to:

(a) The total duty periods to which a crew member is assigned shall not exceed:
  (1) 60 duty hours in any 7 consecutive days;
  (2) 190 duty hours in any 28 consecutive days.
(b) The total flight time of the flights on which an individual crew member
    is assigned as an operating crew member shall not exceed:
  (1) 100 flight hours in any 28 consecutive days;
  (2) 900 flight hours in any calendar year;
  (3) 1000 flight hours in any 12 consecutive calendar months.


text:
What is the difference between HFEC and LFEC inspections?
Sometimes we have to do High Frequency Eddy Current (HFEC) or Low Frequency Eddy Current (LFEC) inspections. My question is: what are the major differences between these two types of inspection? Which one is preferable and more economical? Which type of inspection is required in which cases?

In general, higher frequency gives better resolution of potential defects but with lower penetration into the material (depth). So HFEC is used to detect surface defects and near-surface defects, but LFEC will be required to detect deeper defects.
From this page on the NDT Resource Center website:

The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration, they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d), the eddy current density is down to only 5% of the surface density.
  Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a flaw indication.


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Is the "glass nose" Tu-134 design for easier conversion to military use?
Early versions of the Tu-134 had a glass ('glazed') nose that is unusual for a passenger airliner. It was removed from the later versions.

Source: wikimedia.org
How probable is the hypothesis that this early design was for the easy conversion of Tu-134s into bomber? Glass noses are common in bombers.

Someone (I think Peter Kämpf) mentioned on aviation.SE that that was indeed the case with one of the Russian airliners but I can't find it at the moment.

From wikipedia: The lineage of early Soviet airliners could be traced directly to the Tupolev Tu-16 strategic bomber, and the Tu-134 carried over the glass nose for the navigator

@TomMcW: Here: Older airliners from the Fifties and Sixties were designed with dual use in mind, and many of them had a glazed nose …

It's because of its lineage, based on military aircraft. The Tu-134 provided a glass nose for a navigator position, providing them with a clear view of the ground ahead.
According to Tupolev Bureau, Tu-134 is an upgrade of Tu-124:

Tu-134 ... is an profound upgrade of Tu-124 with turbofan engines, arranged in the aft part of the fuselage.

Tu-124 from tupolev.ru
Tu-124, in its turn, was based on the Tu-104:

In 1958, designing of such passenger plane was started in the Tupolev Design Bureau. Tupolev’s proposal for creation of a new short-range passenger aircraft on the basis of the common aerodynamic and structural layout of Tu-104 has been worked out ...

Tu-104 from tupolev.ru
In turn, Tu-104 was based on the Tu-16 Badger, which meant that it inherited the glazed nose:

In the end of 1953, the Design Bureau’s leaders, headed by A.N. Tupolev, basing on the positive experience of developing, testing and beginning of serial production of Tu-16, proposed the idea of creating jet passenger aircraft on its basis to the leadership of the country.

and

On June 11, 1954 the Council of Ministers of the USSR issued a decree № 1172-516 on creation a long-range high-speed passenger aircraft designated Tu-16P .. . It was to be designed on the basis of Tu-16 airplane with two AM-3-200 engines.

"Tu-16" by No machine-readable author provided. Jno~commonswiki assumed (based on copyright claims). - No machine-readable source provided. Own work assumed (based on copyright claims).. Licensed under CC BY 2.5 via Commons.
The glazed nose had a practical purpose- the navigator sat there (the aircraft needed a crew of three). The later versions of Tu-134 (Tu-134B) on the other hand, did away with the glazed nose.

Image from airliners.net
The Tu-334, on the other hand, had a completely new design, eliminating the glazed nose.

I am not sure lineage is enough to explain the glass nose, because it was not carried over from Tu-16 as is. It had more to do with how navigators were used to work in Soviet Union (relying more on ground references, because navaids were in short supply in Siberia).

This answer seems to be backwards. Surely, the point is not that plane A had one because it was based on plane B, which was based on C, which was based on D, which had a glass nose and, what? they were too lazy to design a new nose? It's surely harder to build a glass nose and they would have only kept it because it was useful.

I agree with @DavidRicherby - Making a new nose would have been almost no trouble whatsoever.

@JanHudec Navigation has been cited as the reason for having a glazed nose in a number of places, though I'm not able to find anything official. I'm not very sure this might be reason- It's quite difficult to navigate visually in Siberia (or any other remote place) in poor visibility conditions.

Converting a Tu-104, a Tu-124 or a Tu-134 to a bomber only would had worked if there had also been provisions for a bomb bay. Including this in a pressurized fuselage would had incurred a severe weight penalty, so there was no easy way for a bomber conversion. This should make the speculation about dual use as a bomber moot.
However, there are several good reasons to use the glazed nose:

Easier conversion of trained bomber crews (trained on the Il-28 and Tu-16) for civilian duties. Remember that the Tu-104 was the second jet airliner in service and the only operational jet airliner worldwide between 1956 and 1958.
Operation as a military transport on improvised flights (where the better view of the navigator in the nose would help in visual navigation).
Civilian operation on flights on routes without navigation aids.

The last point is added by @JanHudec as a comment to this answer, but would exclude the operation on a scheduled service, because the aircraft would not be able to navigate in fog or at night.
Since the glazed nose was removed quite early in the production run of the Tu-134 (from the A-2 version on), it seems that the first reason (dual use of Tu-16-trained crews) is the most likely. While more than 1,500 Tu-16s were built, the Tu-104 (201 built from 1956 to 1960) and Tu-124 (164 built from 1960 to 1965) were relatively rare. From the Wikipedia page on the Tu-104:

Pilots with previous Tu-16 experience transitioned into the Tu-104
  with relative ease

another oft mentioned advantage of the glass nose was that the aircraft could easily be fitted with cameras to operate as spy planes during Aeroflot flights over western Europe, and I remember hearing of several cases where Aeroflot 134s and 154s with glass noses were investigated and cameras found after having "gotten lost" on their regular routes and "just happened" to fly over NATO installations. Whether those were factual or not I don't know, 30+ year old memories...


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Why do the outer parts of wings generate less lift?
In this Wikipedia page it says:

Constant chord: parallel leading & trailing edges. Simplest to make, and common where low cost is important, e.g. in the Piper J-3 Cub but inefficient as the outer section generates little lift while adding both weight and drag.

Why does the outer section generate less lift?

The outer wing section generates less lift because flow around the wingtips reduces the pressure difference between both sides. This effect dies down quickly when moving inboard, but will still reduce lift somewhat at the wing root.

Screenshot of the Stanford Aero wingcalc page. This is a Java page for which you need to add http://aero.stanford.edu/wingcalc.html to the exceptions list in Java. Then you can change aspect ratio, wing sweep, taper ratio and twist. Here the lift distribution for an unswept, untwisted rectangular wing is shown which has a local lift coefficient of almost 1 at the center. Due to the low aspect ratio, wing efficiency is still at 99%
A rectangular wing can be optimal if stall characteristics and ease of construction are included:

The lower lift coefficient at the tip ensures that the stall starts at the wing root, resulting in a docile stall behavior without a sudden rolling motion.
The constant chord allows to use one single jig for building all ribs of a wooden wing.
The shorter root chord of a rectangular wing requires a heavier spar, but when wings are braced and the root bending moment is low, the weight impact is negligible.

If the goal is a simple to build aircraft with forgiving stall characteristics, the rectangular wing will be an efficient choice, especially when bracing is used. Only when performance is most important will a rectangular wing be less efficient than a trapezoidal wing.

From the Wikipedia page's sources, it appears that aerodynamics is the main reason that outer parts generate less lift.
http://www.flyingmag.com/rectangular-wings

The second apparent advantage is aerodynamic, and has to do with the drag penalty associated with lift,the so-called induced drag. Induced drag cannot be eliminated, but it is at its minimum when the spanwise distribution of lift is elliptical. Equating area distribution with lift distribution (a big conceptual leap, but let's make it for the time being), the argument goes on to assert that a rectangular wing deviates greatly from the elliptical ideal, especially near the tips, whereas a moderately tapered wing or a double-tapered one, with a straight inner section and a tapered outer section approximates the ellipse much more closely.

This Wiki page on Lift-induced drag describes how rectangular wings produce stronger wingtip vortices, which in turn lead to less effective lift generation.
https://en.wikipedia.org/wiki/Lift-induced_drag

When producing lift, air below the wing is generally at a higher pressure than the air pressure above the wing, while air above the wing is generally at a lower than atmospheric pressure. On a wing of finite span, this pressure difference causes air to flow from the lower surface wing root, around the wingtip, towards the upper surface wing root. This spanwise flow of air combines with chordwise flowing air, causing a change in speed and direction, which twists the airflow and produces vortices along the wing trailing edge. The vortices created are unstable, and they quickly combine to produce wingtip vortices.
Wingtip vortices modify the airflow around a wing. Compared to a wing of infinite span, vortices reduce the effectiveness of the wing to generate lift, thus requiring a higher angle of attack to compensate, which tilts the total aerodynamic force rearwards.
--
A rectangular wing produces stronger wingtip vortices than a tapered or elliptical wing, therefore many modern wings are tapered.
Similarly, for a given wing area, a high aspect ratio wing will produce less induced drag than a wing of low aspect ratio because there is less air disturbance at the tip of a longer, thinner wing. Induced drag can therefore be said to be inversely proportional to aspect ratio.

This NASA study goes into detail mathematically and describes why the spanwise distribution of lift must be elliptical to minimize drag.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760012005.pdf

"...when the spanwise distribution of lift is elliptical" This is exactly the crux of my question. Why is this the case?

@ayomeer I think this NASA study answers your question quite well. https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19760012005.pdf

Well, this study is from NACA and proves that a nearly triangular lift distribution causes the lowest induced drag. Read here for more.

" Therefore, reducing lift at the tips and adding more lift at the root will create a lighter wing for a modest drag increase, resulting in an overall optimum for an almost triangular lift distribution. When compared to an elliptical wing planform, the total wing span of such an optimized wing is bigger for the same overall drag, but this wing will weigh less."


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What is the relationship between blade inertia and rotor kinetic energy?
As far as I know:

High blade inertia provides good pilot intervention time (the time
between recognizing engine fail and pushing the collective down).
High kinetic energy allows for a soft touch down after flare in
autorotation.

What is the relationship between blade's inertia and rotor kinetic energy?

is this a question about dynamics/kinematics (physics) or there is an actual aviation side of the question?

@Federico many thanks, I edited my question

point c seems a question, not "as far as you know".

The relationship between blade inertia and rotor kinetic energy is:
$$K = N\frac{1}{2}I\omega^2$$
$$I = \int mr^2dr$$
Where:

$N$ is the number of blades
$I$ is the moment of inertia of a blade
$ω$ is the rotational velocity (rotor rpm)
$m$ is mass
$r$ is distance from hub along the blade

For a fixed RPM increasing the moment of inertia of the blade increases kinetic energy of the rotor. An increase of the moment of inertia can be obtained by increasing the mass of the blades (inefficient, rotor gets heavy and requries more power from engine, less payload, etc.) or by placing relatively small masses at the tip of the blade.

should $m$ be $m(r)$?

@falstro: Absolutely, the mass distribution may change along the blade. I avoided to add the variable to keep the formula less cluttered.

@Emil, the formula shows us if blade's inertia is high the rotor kinetic energy should be high. Another word blade inertia and rotor kinetic energy are two different terms for the same thing. By the way I referred the Prouty's book. The blade inertia unit is (slug-feet) and rotor kinetic energy unit is (foot-pounds) Am I right?

It is possible to have a low Inertia $I$, but combined with a high rotational velocity $\omega$ this will still give a high kinetic energy.

A simple "no math" answer:
The higher the inertia, the more it can "resist" anything that wants to change its velocity.

a. High blade inertia is good pilot intervention time ( the time between engine fail and collective down which army requirement is 2 second.

It usually takes the pilot a few seconds to realize things are going wrong.  Call it the "WTF!" moment.  A high inertial rotor system will see less decay in speed during those moments.  A successful auto-rotation requires preserving that rotor RPM.

b. High kinetic energy is good for soft tough down after flare in autorotation.

So you made it past the "WTF", kept the rotor speed up, picked out your landing spot, and are in full auto-rotation.  As you approach the ground you now are going to need to "flare" - increase collective and haul back on the cyclic.  The higher inertia rotor system allows for a "sloppier" flare because the inputs (to use the rotors stored energy to arrest the descent) will have less effect on the RPM.

c. What is the relationship between them?

In both cases, the higher inertia system works in the pilots favor.
An extreme example of a high inertia rotor is the (never ending) Carter-Copter Gyro project.  They have "weighted" tips to increase rotor inertia.
Older video, but jump takeoff demo'd around 40 sec mark.  The rotor on an auto-gyro can be "pre-rotated" with some assist from the engine, but when he makes that jump, that is all inertia working in his favor.

Welcome to Aviation.SE! This is a very nice answer, although some links or sources would make it even better. Perhaps you might consider creating a user account and adding your own questions as well.


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Would connecting propeller blades with a continuous ring reduce induced drag?
Turbine engines are covered, which of course is to contain the process (just like a super/turbocharged engine is as soon as the air enters the intake). But it got me thinking, doesn't this also reduce - or even completely remove - the induced drag around the tips of the fan blades?
Would it be possible to have a similar setup for a standard prop, it doesn't even have to be stationary, it could be a ring connecting the tips of the prop, spinning with it, like an infinite winglet.  It has the added safety benefit that it'll be visible when the prop is spinning. And I figure if the ring is strong enough to maintain its circumference, the load on the prop should be marginal since it's spinning around its own center of mass.
Is the induced drag on the prop not large enough to warrant any thought, or would such a prop-winglet-ring (I'm sure there's a real name for it, anyone know what I'm talking about?) cause other disruptions of the airflow? Or perhaps there are other reasons, like it would simply be too hard to solve for constant-speed props?

There are already plenty of answers below that solve induced drag on props. To directly answer your question regarding a ring attached directly to the blades - it would suffer variable centrifugal loads, which by themselves would be rather large on high RPM, but the fact that the loads would vary so much would lead to material fatigue and failure.

How would you connect the tips of the propeller's blades with a continuous ring without rendering them incapable of changing pitch?

What you're talking about does exist, they are called Q-Tip Propellers.
Remember that a propeller blade is just an airfoil - like a wing - and the basic aerodynamics are no different than a wing. But the rotation of the blade creates more phenomena than a wing, in particular the helicoidal vortex one sees behind the prop and causes all sorts of propeller effects.
In theory, nothing would prevent us from having winglets at the prop tips :
the advantages would be

Making the prop more efficient by reducing the induced drag (same as a winglet on a wing )
Reducing noise
Keeping the propeller tip speed subsonic by decreasing its length

The big problem is in aerodynamic stresses, and as far as I know there have been some quite spectacular failures during testing, so the solution is now to give a greater sweep to the tips (see that as equivalent to the 777 wing tip compared to the 787 for instance). Try and find articles on the Hartzell Q-tip.
As naval propellers are wider and capable of deealing with bigger torque stresses, modern ones do have winglets. You could find some pictures on the web.

Cool! Do you know if there are any 'rings' which connect all prop-tips, all the way around? Or put a frame around the prop, like on some water vehicles, and some helicopters (sci-fi at least, not sure about real ones)?

A related concept to the Q-Tip propeller (with fewer issues due to aerodynamic stress) is the Scimitar Propeller, which is found on many turboprop aircraft including the C-130J "Super Hercules". Though most of the scimitar propellers I'm familiar with are constant-speed I believe there are some fixed-pitch variants as well...

As others have pointed out, fitting the ring to the prop will greatly increase the stress on the blades. The same effect can be had with a nicely fitted shroud.
There has indeed been a plane which used this concept, the RFB FanTrainer (see picture below). To reduce weight and wetted area, the prop diameter was much smaller than with a regular propeller, so the overall efficiency was not better. However, the smaller rotating inertias did produce a more turbine-like effect (less precession), so the concept was used for a basic trainer for future jet pilots.

In the end, the FanTrainer enjoyed only limited success and was discontinued after 50 had been built. The design was too lightweight to support all the desires of air forces for a basic trainer, and the private market at that time was shrinking and full of older planes which served the cost-conscious customers equally well. It did, however, offer almost jet-like characteristics for a uniquely low price per flight hour.
In general, if you want to shroud the propeller for better efficiency, you need to accept the higher surface area of the shroud, which will quickly add more drag than you are ever likely to save by preventing flow around the prop tips.
What could be saved by shrouding the prop? Induced drag would be the same, since this comes from lift creation. The classical theory for minimum induced loss propellers by A. Betz and L. Prandtl requires an elliptic lift distribution over the propeller disc, such that lift smoothy tapers off at the tips. Artificially increasing it would only help if this could reduce blade chord at the tips - since the tips see the highest dynamic pressure, this could indeed translate into less friction drag. However, this gain is small when compared to the massive increase in friction drag of a shroud.
At high speeds the induced losses are small, and other factors become dominant. Note that turbofans and highly loaded propellers are not designed for minimum induced loss, but for maximum thrust with a given diameter. A shrouded propeller can enjoy a higher disc loading, so you get the same thrust with smaller blades and lower tip speeds, which will help in high speed efficiency. Smaller blades translate into less friction losses on the prop, and lower tip speeds translate into higher cruise speed before Mach losses begin to bite.
Thus, at high speed a shroud can be helpful when it is not too large. Turbofan engines suffer from this dilemma. They could have much higher bypass ratios than today, but this would mean huge nacelles, and the increased nacelle drag would offset the gains from the increased bypass ratio. Actively laminarising nacelle flow is the way forward here, but so far the practical implementation has yet to happen.

On the subject of rotating inertia, I imagine a rotating shroud would also have a high angular momentum, leading to gyroscopic effects whenever the orientation of the propeller disc changed. In addition to affecting the handling of the airplane, I think this would impose a cyclic bending load on the propeller blades during the maneuver, and I can imagine the arrangement being prone to wobble.

@sdenham: Yes, letting the shroud rotate with the propeller will bring a lot of problems. It is better to keep it fixed, like on turbofans.

@PeterKämpf - you touch upon tip speeds, and just wave drag, but what about thrust augmentation? Certainly that was a key goal in early designs like the various lifting platforms.

@MauryMarkowitz Thrust augmentation needs some forward-facing area of the shroud for suction to work on. The lifting platforms had that, but prop shrouds offer very little because they are designed to operate at high forward speed. In other words, the low vertical speeds of lifting platforms enable those to make use of thrust augmentation, but the high flight speed of prop shrouds shift the optimum to leave litte opportunity for thrust augmentation.

Does the FanTrainer would suffer from complex high precision/complex process to balance the ring/shroud?. I ask since the engine is in the front not after the prop. Or that doesn't matter.

@Gabe The shroud is a static part. Only the fan inside rotates. Gyro effects are much smaller due to the smaller fan diameter. If you mean by "balance" how to achieve a small, consistent gap between fan and shroud: I have not enough detail knowledge to answer this, but judging from turbofan engines, it is not easy but doable.

@PeterKämpf yes that's what I meant (consistent gap)

A ducted fan comes close to what you're describing, although the ring around the propeller is stationary instead of attached to and spinning with the propeller.
The main advantage of a ducted fan is higher efficiency due to reduced propeller blade tip losses (essentially induced drag) but this efficiency advantage is lost at higher speeds and/or lower thrust demand.
In "normal" aircraft, the drawbacks of a ducted fan outweigh the efficiency gains. Ducted fans are mainly used in airships and VTOL aircraft like the infamous Bell X-22. They are also used in most jet model airplanes.

Right ducted fan was what I was searching for when I was mentioning the 'sci-fi' choppers, thanks! :)

Q-Tips and Ducted fans are the big ones to solve the problems you're thinking of.
Your ring idea would be very difficult to implement for a number of reasons, weight being primary. A metal ring all the way around the propeller would add a significant amount of weight to the aircraft, which would likely cancel out any efficiency gains you get from stabilizing the airflow. In addition, the tips of a propeller are already experiencing several  thousand G's at normal operating RPMs. This is acceptable because the prop gets continuously lighter as you approach the tips. But if you were to attach a ring of metal weighing a few dozen pounds, the forces would be astronomical, and your prop would very quickly fail.
A second issue is that in order to have efficient propellers, we rotate the blades slightly to change the angle at which they bite into the air. These are called constant speed propellers, and they're already somewhat complicated. If you go and add a second pivoting point to the prop tips so that they can move inside the ring, you're just adding a bunch of bearings, grease, weight, and another failure point.
Finally, balancing the ring would likely be a difficult task. First your ring would have to be manufactured to very precise tolerances which would be quite expensive. The slightest nick or dent in the ring (which happens frequently to propellers) will cause it to become unbalanced and would at minimum require work, and at most could cause the whole propeller to shake itself apart. This is already a minor concern for props, but when you put your heavy disk out on a long arm from it's fulcrum, then subject it to incredibly high G forces, you're just amplifying any faults it may have.

For historical perspective do some research on the Culver Channel Wing produced in 1952-53.  This twin engine (pusher) plane has two ducts that do not completely encircle the prop but are part of the wing.  This lead to extremely short take off capabilities because the airflow over the wing was not tied to forward ground speed. I would even go as far to say that it was an early step in ducted fan VTOL capabilities.
This article by Doug Robertson posted in 2005 on airport-data.com contains some beautiful pictures, and an what appears to be a well researched narrative history of the aircraft.